Light-weight vehicles are being subjected to a growing and significant problem, Explosively Formed Projectiles (EFPs). EFPs are highly dense solid matter traveling at 7,000 to 8,000 fps with very high kinetic energy making it much harder to stop using a flying plate method.
Even more problematic are weapons, such as anti-tank rounds, that are shape-charges that create high-velocity molten jets with a tip velocity of about 9,000 meters per second (mps). These rounds use a conical shape charge capable of producing a high temperature jet delivering a tremendous amount of energy on a single point. Such weapons can defeat most types of armor.
Stopping a Projectile
The basic concept in stopping a projectile is that work must equal energy. The more work the armor can do on the projectile, the more kinetic energy it can absorb. Conventional armor augments work by increased frictional force through hardness, tensile strength and thickness of the armor system.
Normal force is what gives rise to the friction force, the magnitudes of these forces being related by the coefficient of friction “μ” between the two materials:f=μNTherefore, given the mass and velocity of the projectile a simple equation would define the thickness “d” and “f” force to stop the projectile. See Diagram 1.
The hydrodynamic impact of an EFP or a shape charge delivers an enormous amount of energy. In the past, stopping an EFP has been directly related to the density of the armor. It has always been a balance between weight and thickness. The current solution of using rolled homogeneous armor (RHA) backing with Polyethylene and other composites is not a viable solution for light-weight vehicles. For example, to defeat a medium EFP the required armor would be 12-16 inches thick and 80-120 lbs/psf. Using this logic to stop the large threat the armor system would need to be more than 21 inches thick.
Conventional reactive armor systems produce significant back pressure and lethal secondary fragments. When designing a proactive armor for light-weight vehicles, minimizing back pressure as well as harmful secondary fragments are major factors to consider.